Painted steel sheet provided with a zinc coating

ABSTRACT

A steel sheet is provided with a coating having at least one layer of zinc and a top layer of paint applied by cataphoresis. The zinc layer is deposited by a jet vapor deposition process in a deposition chamber maintained at a pressure between 6·10−2 mbar and 2·10−1 mbar. A fabrication method and an installation are also provided.

This is a Continuation of U.S. application Ser. No. 16/381,488 filed onApr. 11, 2019, published as U.S. 2019/0233934 A1, which is a Divisionalof U.S. application Ser. No. 14/908,015, filed Aug. 2, 2016, patented asU.S. Pat. No. 10,400,326 on Sep. 3, 2019, which is a National Phaseapplication of International Patent Application PCT/IB2013/001682, filedAug. 1, 2013, the entire disclosures of which are hereby incorporated byreference herein.

This invention relates to a steel sheet provided with a coatingcomprising a layer of zinc covered by paint, which is intended inparticular for the fabrication of automobile parts, although it is notlimited to that application.

BACKGROUND

Galvanized coatings comprising essentially zinc are conventionally usedfor the effective protection they provide against corrosion, whether inthe automotive sector or in the construction industry, for example.

In the following text, a zinc coating means a coating of pure zinc,potentially including the unavoidable impurities acquired duringproduction and present in trace quantities.

The sheets coated in this manner can then be cut and shaped, for exampleby stamping, bending or shaping, to form a part that can then be paintedto form a paint film on top of the coating. This paint film is generallyapplied by cataphoresis.

The methods most frequently used to deposit a zinc coating on thesurface of a steel sheet are galvanizing and electrogalvanizing.However, these conventional methods do not make it possible to coatgrades of steel that contain high levels of oxidizable elements such asSi, Mn, Al, P, Cr or B, which has led to the development of new coatingmethods, and in particular vacuum deposition technologies such as jetvapor deposition (JVD).

BRIEF SUMMARY OF THE INVENTION

Nevertheless, the surfaces of the sheets coated according to thesevacuum deposition methods, following the step of painting bycataphoresis, exhibit surface defects that adversely affect theaesthetic appearance of the shaped parts.

An object of the present invention is therefore to eliminate thedisadvantages of steels coated using methods of the prior art by makingavailable a steel sheet coated with zinc by vacuum deposition and alayer of paint that has a good surface appearance.

The present invention provides a steel sheet. The steel sheet has acoating with at least one layer of pure zinc and potential unavoidableimpurities acquired during production and present in trace quantities,and a top layer of paint applied by cataphoresis. The zinc layer is thetop layer of the coating before the application of the paint layer andthe zinc layer is deposited by a jet vapor deposition process in adeposition chamber maintained at a pressure P_(chamber) between 6·10⁻²mbar and 2·10⁻¹ mbar.

The sheet can also have the following characteristics, consideredindividually or in combination:

-   -   the steel sheet is obtained by a method wherein the ratio        between the pressure inside the deposition chamber P_(chamber)        and the pressure inside the zinc ejection chamber P_(eject) is        between 2·10⁻³ and 5.5·10⁻²;    -   the steel sheet is obtained by a method wherein the distance d        between the upper portion of the slot 8 of the ejection chamber        7 and the steel sheet to be coated is between 20 and 60 mm;    -   a surface of the steel sheet has no more than 2.7 crater-type        defects per square decimeter; and    -   the coated steel is a Very High Strength steel.

The present invention further provides a method for the fabrication of acoated and painted sheet. The method includes the steps of the coatingthe sheet by a sonic vapor jet of zinc inside a deposition chambermaintained at a pressure P_(chamber) between 6·10⁻² mbar and 2·10⁻¹mbar.

The method can also have the following characteristics, consideredindividually or in combination:

-   -   the ratio between the pressure P_(chamber) inside the deposition        chamber and the pressure P_(eject) inside the ejection chamber        is between 2·10⁻³ and 5.5·10⁻²; and    -   a distance d between an upper portion of the slot 8 of the        ejection chamber 7 and the steel sheet to be coated is between        20 and 60 mm.

Additional characteristics and advantages of the invention are describedin greater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

To illustrate the invention, tests have been conducted and will bedescribed by way of non-restricting examples, in particular withreference to the accompanying figures, in which:

FIG. 1 illustrates a jet vapor deposition installation that can be usedto carry out the method claimed by the invention;

FIG. 2 is a photograph at an ×4 enlargement of a sheet coated accordingto the prior art; and

FIG. 3 is a photograph at an ×4 enlargement of a sheet coated accordingto the invention.

DETAILED DESCRIPTION

The sheet coated according to the invention first comprises a steelsubstrate, preferably hot-rolled then cold-rolled so that it can be usedfor the fabrication of automobile body parts. The invention is notlimited to this field, however, and can be used for any steel partregardless of its intended final use.

The steel substrate can in particular be one of the following grades ofa VHS (Very High Strength steel, generally between 450 and 900 MPa) orUHS (Ultra High Strength, generally greater than 900 MPa) steel thatcontain high levels of oxidizable elements:

-   -   steels without interstitial elements (IF, Interstitial Free),        which can contain up to 0.1% by weight Ti;    -   dual-phase steels such as DP 500 steels, up to DP 1200 steels,        which can contain up to 3% by weight Mn in association with up        to 1% by weight Si, Cr and/or Al,    -   TRIP (TRansformation Induced Plasticity) steels such as TRIP 780        steel, which contains, for example, approximately 1.6% by weight        Mn and 1.5% by weight Si;    -   TRIP steels or dual phase steels containing phosphorus;    -   TWIP (TWinning Induced Plasticity) steels with a high content of        Mn (generally 17-25% by weight),    -   low-density steels such as Fe—Al steels, which can contain up to        10% by weight Al, for example;    -   stainless steels, which have a high concentration of chromium        (generally 13-35% by weight), in association with other alloy        elements (Si, Mn, Al etc.).

The steel sheet can optionally be coated with one or more additionallayers in addition to the zinc layer in a manner appropriate to thedesired properties of the final product. The zinc layer will preferablybe the top layer of the coating.

A method for the fabrication of the steel sheet according to theinvention is illustrated more particularly in FIG. 1 , which shows aninstallation 1 that comprises a vacuum deposition chamber 2. Thischamber comprises an entry lock and an exit lock (not shown), betweenwhich the steel sheet 3 to be coated circulates. The sheet 3 can bemoved by any appropriate means, for example a rotating support roller onwhich the strip can be supported.

Situated facing the surface of the strip to be coated is an ejectionchamber 7 equipped with a slot 8, the upper part of the slot 8 beingsituated at a distance d from the surface of the strip to be coated, offor example, between 20 and 60 mm. This chamber 7 is mounted on anevaporation crucible 4 that contains the liquid zinc 9 to be depositedon the surface of the steel strip 3. The evaporation crucible 4 isadvantageously equipped with an induction-heating device 5 that makespossible the formation of the vapor. The vapor then escapes from thecrucible via conduit 10 that conducts it to the ejection chamber 7 andthe slot 8, which is preferably calibrated to form a jet directed towardthe surface of the substrate to be coated. The presence of the slot 8allows for the regulation of the mass flow of vapor, at a constant sonicspeed along the slot (sonic throat) that has the advantage of achievinga uniform deposit. Reference to this technology is made below, using theacronym “JVD” (for Jet Vapor Deposition). Additional information on thistechnology is presented in patent EP07447056.

In another embodiment not illustrated, the crucible and the ejectionchamber are one and the same part, comprising a slot directed toward thesurface of the substrate to be coated. In this embodiment, the vaporcreated by heating the zinc rises directly toward the slot and forms ajet directed toward the surface of the substrate to be coated.

The pressure P_(chamber) in the deposition chamber 2 is maintained at apressure between 6·10⁻² mbar and 2·10⁻¹ mbar.

The pressure P_(chamber) in the deposition chamber 2 and the pressureP_(eject) in the ejection chamber 7 are optionally maintained so thatthe ratio P_(chamber) to P_(eject) is between 2·10⁻³ and 5.5·10⁻², whichallows for the improvement of the temporary protection of thesecoatings.

A layer of oil is optionally applied to the surface of the sheet thuscoated to provide temporary protection when the sheet is stored in a wetand/or saline environment before delivery or the transformation into thefinal product.

The sheet 1, which may or may not have been subjected to a skin-passstep, can then be cut and shaped, for example by stamping, bending orshaping, to form a part that can then be painted to form a paint film onthe coating.

For automotive applications, after a phosphate treatment, each piece isquenched in a cataphoresis bath and a layer of primer paint, a layer ofbase paint and optionally a finish varnish coat are applied insuccession.

Before applying the cataphoresis layer to the part, the part is firstde-greased then phosphatized to ensure the adherence of the cataphoresislayer.

The cataphoresis layer provides additional protection for the partagainst corrosion. The layer of primer paint, generally applied with apaint gun, prepares the final appearance of the part and protects itagainst grit and against UV radiation. The base paint layer gives thepart its color and its final appearance. The varnish layer gives thesurface of the part good mechanical strength, resistance to aggressivechemical agents and a good surface appearance.

Generally, the weight of the phosphate layer is between 1.5 and 5 g/m².

The paint films used to protect and guarantee an optimal surfaceappearance of the parts comprise, for example, a cataphoresis layer witha thickness of 15 to 25 μm, a layer of primer paint with a thickness of35 to 45 μm and a layer of base paint with thickness of 40 to 50 μm.

In cases where the paint films also comprise a layer of varnish, thethicknesses of the different layers of paint are generally as follows:

-   -   cataphoresis layer: between 15 and 25 μm, preferably less than        20 μm,    -   layer of primer paint: less than 45 μm,    -   layer of base paint: less than 20 μm, and    -   layer of varnish: less than 55 μm.

The paint films can also not comprise a cataphoresis layer and compriseonly one layer of primer paint and one layer of base paint, andoptionally a layer of varnish.

Preferably, the total thickness of the paint films will be less than 120μm, or even less than 100 μm.

Sometimes on the surface of the sheet following the application of thecataphoresis layer, crater-type defects are observed which, on steelsheets, are privileged sites for the origin of corrosion andsignificantly degrade the appearance of the surface of the sheet. Thesecraters are in the form of truncated conical holes that emerge in thesurface of the cataphoresis layer and can possibly extend through thecoating to reach the surface of the steel substrate; they generally havea diameter between 100 and 500 μm at the base and between 5 and 20 μm atthe summit.

The invention will now be explained below on the basis of testsperformed by way of non-restricting examples.

Tests Acceptance Criteria

To evaluate the sensitivity of the product to the risk of the appearanceof crater-type defects, there is a criterion relative to the number ofdefects present on a coated steel sheet 10 cm×15 cm, after this sheethas been subjected to polishing. For the coated steel sheet to beaccepted, it must have fewer than four defects per 10×15 cm² plate,which is equivalent to less than 2.7 defects per square decimeter.

Tests

3 series of cold-rolled IF steel sheets, type DC06, of the type marketedby ArcelorMittal, having a zinc coating 7.5 μm thick were built.For both specimens, the coating was applied by JVD deposition at adifferent pressure in the deposition chamber, with a distance d betweenthe upper part of the slot of the extraction [sic; ejection] chamber andthe surface of the identical strip to be coated equal to approximately35 mm.

Specimen Type of coating 1 JVD - pressure <10⁻² mbar in the depositionchamber 2* JVD - pressure 1.1 · 10⁻¹ mbar in the deposition chamber*According to the invention

The specimens were then coated with Quaker Ferrocoat oil N 6130 at 1.2g/m²±0.3 g/m², and then subjected to the phosphate and then cataphoresissteps. An image capture and processing device such as the commerciallyavailable TalySurf CLI 2000 then made it possible to calculate thenumber of crater-type defects as defined above present on the surface ofthe coated strip. These craters are in the form of truncated conicalholes that emerge in the surface of the cataphoresis layer and canpossibly extend through the coating to reach the surface of the steelsubstrate.

Specimen Number of defects 1 >>>2.7 dm² [sic; 2.7/dm²] (up to 1600/dm²)2* <2.7/dm²

Specimen No. 2 according to the invention therefore satisfies theacceptance criterion, in contrast to specimen No. 1.

FIG. 2 is an ×4 enlarged photograph of a steel sheet of the prior art towhich a coat of paint has been applied using a cataphoresis process.This sheet of cold-rolled IF steel sheet of DC06 was coated with 7.5 μmof Zn using a JVD process in which the pressure in the depositionchamber was maintained at a pressure of less than 10⁻² mbar, thedistanced being equal to 35 mm. The sheet coated in this manner wascoated with a layer of Quaker Ferrocoat oil N 6130 at 1.2 g/m²±0.3 g/m²to provide a temporary protection of the surface, and was then subjectedto a cataphoresis-painting step. Crater-type defects 11 as defined abovewere observed on the surface of this sheet. These defects significantlydegrade the appearance of the surface of the sheet.

FIG. 3 is an ×4 enlarged photograph of a steel sheet according to theinvention. This sheet of cold-rolled IF steel sheet of DC06 was coatedwith 7.5 μm of Zn, using a JVD process, whereby the pressure in thedeposition chamber was maintained at a pressure of 1.1·10⁻¹ mbar, thedistanced being equal to 35 mm. The sheet coated in this manner wascoated with a layer of Quaker Ferrocoat oil N 6130 at 1.2 g/m²±0.3 g/m²to provide a temporary protection of the surface, and was then subjectedto a cataphoresis-painting step. The absence of crater-type defects onthe surface of this steel sheet is apparent. The shades of gray thatappear in the figure are related to the roughness of the surface of thesteel sheet and are not related to defects in the sense described above.

The same results can be observed with the use of a Fuchs Anticorite oilRP 4107s at 1.2 g/m² instead of the Quaker Ferrocoat.

The inventors have also noted that the change in pressure inside thedeposition chamber does not affect the rate of deposition of the coatingon the surface of the steel sheet.

What is claimed is:
 1. A coated steel sheet, the coating comprising: atleast one layer of zinc, wherein the at least one layer of zinc includesa layer of pure zinc and unavoidable impurities acquired duringproduction and present in trace quantities; and a top layer of paintapplied by cataphoresis; the at least one layer of zinc being the toplayer of the coating before the application of the paint layer, the atleast one layer of zinc being a layer deposited by a jet vapordeposition process in a deposition chamber maintained at a pressureP_(chamber) between 6·10⁻² mbar and 2·10⁻¹ mbar.
 2. The coated sheetaccording to claim 1, wherein the jet vapor deposition process furtherincludes maintaining an ejection chamber that is located inside thedeposition chamber at a pressure P_(eject), a ratio of the pressureP_(chamber) to P_(eject) being between 2·10⁻³ and 5.5·10⁻².
 3. Thecoated steel sheet according to claim 1, wherein the coated steel is aVery High Strength steel.
 4. A method for the fabrication of a coatedand painted sheet comprising the steps of: providing a sheet in adeposition chamber; maintaining a pressure P_(chamber) inside adeposition chamber between 6·10⁻² mbar and 2·10⁻¹ mbar; and coating thesheet with a sonic vapor jet of zinc inside the deposition chamber, saidzinc consisting of pure zinc and unavoidable impurities acquired duringproduction and present in trace quantities; painting the coated sheet.5. The method according to claim 4, further comprising the step of:maintaining an ejection chamber that is located inside the depositionchamber at a pressure P_(eject), a ratio of the pressure P_(chamber) toP_(eject) being between 2·10⁻³ and 5.5·10⁻².
 6. The method according toclaim 4, wherein a distance between an upper portion of a slot of theejection chamber and the steel sheet to be coated is between 20 and 60mm.
 7. The method according to claim 4, wherein said painting includesapplying a top layer of paint over the zinc coating by cataphoresis. 8.The method according to claim 4, wherein said painting includes applyinga cataphoresis layer.
 9. The method according to claim 8, wherein saidpainting includes applying a phosphate layer prior to application of thecataphoresis layer.
 10. The method according to claim 8, wherein saidapplying the cataphoresis layer comprises quenching the sheet in acataphoresis bath.
 11. The method according to claim 10, wherein priorto said quenching, the sheet is subjected to a phosphate treatment. 12.The method according to claim 4, wherein said painting includes applyinga layer of primer paint to the sheet.
 13. The method according to claim10, wherein after said quenching, a layer of primer paint is applied tothe sheet.
 14. The method according to claim 11, wherein the phosphatetreatment applies a phosphate layer having a weight between 1.5 and 5g/m².
 15. The method according to claim 10, wherein said cataphoresisbath provides a cataphoresis layer with a thickness of 15 to 25 μm. 16.The method according to claim 7, wherein the top layer of paint isapplied directly over the zinc coating by cataphoresis.
 17. Aninstallation for coating a steel sheet comprising: a deposition chamberhaving a pressure P_(chamber) maintained between 6·10⁻² mbar and 2·10⁻¹mbar; a steel sheet running through the deposition chamber maintained atsaid pressure P_(chamber); a sonic vapor jet for coating the steel sheetwith at least one layer of zinc inside the deposition chamber, said zincconsisting of pure zinc and unavoidable impurities acquired duringproduction and present in trace quantities; and a cataphoresis bath, atop layer of paint applied to the steel sheet by cataphoresis.
 18. Theinstallation according to claim 17, wherein the at least one zinc layeris a top layer of the coating before the application of the paint layer.19. The installation according to claim 17, further comprising: anejection chamber located inside the deposition chamber, the ejectionchamber having a pressure P_(eject) between 2·10⁻³ and 5.5·10⁻².
 20. Theinstallation according to claim 17, wherein a distanced between an upperportion of a slot of the ejection chamber and the steel sheet to becoated is between 20 and 60 mm.